Methods for production of highly formable extra deep draw enameling steel -- product and process for manufacture thereof

ABSTRACT

A new alloy and process for making highly formable enamel steel substrates. The new alloy is manufactured from a vacuum degassed steel on a Continuous Annealing Line (CAL) with in-line temper equipment and possesses the ductility and enameling properties that make the steel suitable for the manufacture of deep draw and extra deep draw enameling parts. Target applications for the new alloy and process include but are not limited to deep draw and extra deep draw parts such as bathtubs, kitchen sinks, kitchen oven liners and general parts which are press-formed from flat sheets into complicated shapes, which are subsequently vitreously enameled.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/968,459 filed on Mar. 21, 2014, which is incorporated in itsentirety herein by reference.

FIELD OF THE INVENTION

The present invention is related to a method for producing an extra deepdraw enameling steel, and in particular to a method for producing anextra deep draw enameling steel using a continuous annealing line.

BACKGROUND OF THE INVENTION

The use of steels for deep draw and extra deep draw parts such asbathtubs, kitchen sinks, kitchen oven liners and general parts which arepress-formed from flat sheets into complicated shapes and thensubsequently vitreously enameled is known. However, previous industrialtrials for the manufacture of the targeted parts mentioned above usingheretofor known conventional enamel steel grades produced via aContinuous Annealing Line (CAL) have failed due to low ductility. Assuch, conventional enameling steel grades and process routing use abatch anneal process. Yet, conventional enameling steel grades with goodenameling characteristics can still fail in drawn areas when the steelis processed with batch annealing and subsequent skin passingapplications to produce a required enameling surface. Therefore,improved methods for production of highly formable extra deep drawenameling steels and products thereof would be desirable.

SUMMARY OF THE INVENTION

A process for producing highly deformable enamel steel substrates isprovided. The process includes providing a steel slab having a chemicalcomposition in weight percent (wt %) with a range of 0.0100 max carbon(C), 0.0600 max silicon (Si), 0.17 max manganese (Mn), 0.0100 maxphosphorous (P), 0.0200-0.0500 sulfur (S), 0.0150-0.0800 aluminum (Al);0.10 max chromium (Cr), 0.1000 max copper (Cu), 0.0100 max niobium (Nb),0.0500 max molybdenum (Mo), 0.0100 max nitrogen (N), 0.0600 max nickel(Ni), 0.0650-1500 titanium (Ti), 0.0004 max boron (B), 0.0150 max tin(Sn), with the balance being iron (Fe) and incidental impurities. Thechemical composition in wt % of the steel slab also obeys the followingrelationships:

14.0≧[Ti/(C+N)]≧8.0  (1)

and

Ti_(free)=[Ti_((addition))−(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2)

The steel slab is soaked at a temperature of at least 1200° C. and thenhot rolled using a roughing treatment within a temperature range between900-1200° C., inclusive, in order to produce a transfer bar having athickness between 20-70 mm, inclusive.

The transfer bar is hot rolled to produce hot rolled strip using afinishing treatment with an entry temperature between 900-1100° C.,inclusive, and an exit temperature between 760-960° C., inclusive. Thehot rolled has a thickness between 1.5-6.0 mm, inclusive, which can thenbe coiled within a temperature range of 560-790° C. The hot rolled stripis pickled and cold rolled into cold rolled sheet. The cold rolled sheetis annealed using a Continuous Annealing Line (CAL) and the annealedcold rolled sheet has a surface roughness between 2.0-3.5 Ra. Inaddition, the annealed cold rolled sheet has a lower yield strength ofat least 110 MPa, a tensile strength of at least 280 MPa, an elongationto failure of at least 40%, an N-value of at least 0.200, and an R-valueof at least 1.80.

The cold rolled sheet has a reduction in thickness compared to the hotrolled strip of between 50-90%, inclusive. Also, the cold rolled sheetis annealed in the CAL at a soak temperature between 760-880° C. and fora soak time up to 180 seconds before being cooled to an ambienttemperature with a preferred cooling rate of more than 1 K/s andpreferred cooling rate of less than 100 K/s.

In some instances, the process includes interrupting the cooling of thecold rolled sheet from an exit temperature of the CAL furnace to theambient temperature by holding the cold rolled sheet in a temperaturerange between 250-600° C. for a predetermined time. In other instances,the cold rolled sheet is rapidly cooled with cooling rates of more than30 K/s from the soak temperature to the ambient temperature and thenreheated to within the temperature range between 250-600° C. for apredetermined time. In still other instances, the annealed cold rolledsheet is temper rolled between 0.3-2.0%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is graphical plot of lower yield strength (MPa) versus elongationto failure (%) for an inventive steel as disclosed herein (SteelA—circle-shaped data points), a prior art and comparative enamel steel(Steel B—square-shaped data points) and a prior art and comparativeinterstitial free steel (Steel C—diamond-shaped data points);

FIG. 2 is a yield strength (MPa) versus N-value for an inventive steelas disclosed herein (Steel A—circle-shaped data points), a prior art andcomparative enamel steel (Steel B—square-shaped data points) and a priorart and comparative interstitial free steel (Steel C—diamond-shaped datapoints);

FIG. 3 is an optical micrograph for an inventive steel as disclosedherein with arrows pointing to evenly dispersed titanium sulfides withinthe microstructure of the material;

FIG. 4 is an optical micrograph for a prior art and comparative enamelsteel with arrows pointing to evenly dispersed titanium sulfides withinthe microstructure of the material; and

FIG. 5 is an optical micrograph for a prior art and comparativeinterstitial free steel with no titanium sulfides present with themicrostructure of the material.

DETAILED DESCRIPTION OF THE INVENTION

A new alloy and process for making highly formable enamel steelsubstrates is provided. The new alloy is manufactured from a vacuumdegassed steel on a Continuous Annealing Line (CAL) with in-line temperequipment and possesses the ductility and enameling properties that makethe alloy suitable for the manufacture of deep draw and extra deep drawenameling parts. Target applications for the new alloy and processinclude but are not limited to deep draw and extra deep draw parts suchas bathtubs, kitchen sinks, kitchen oven liners and general parts whichare press-formed from flat sheets into complicated shapes, which aresubsequently vitreously enameled.

The inventive alloy and process enables the production of enamelingsteel for deep draw and extra deep draw application on a CAL with andwithout in-line temper rolling. The enameling steel has a high surfacequality and is a highly homogenized cold rolled steel that exceeds themechanical properties provided by conventional Batch Anneal or ColdAnnealing processing routes and is suitable for deep draw and extra deepdraw enameling applications.

The alloy and process disclosed herein provides vitreous enameling steelproduct that fulfills the most stringent strength and ductilityrequirements of steel in this class of material for the manufacturing ofdeep draw and extra deep draw enameling parts. The process and alloyfurther enables the production of high quality and highly ductilevitreous enameling steel utilizing a specialized ultra low carbon (ULC)alloy and a CAL, plus optional in-line temper rolling in order toprovide on-time high quality highly formable steel for the vitreousenameling market.

The specialized enameling alloy and process disclosed herein affords forhigh volume, high quality vitreous enameling steel production with asgood as or better ductility properties than available for similar steelgrades produced via the conventional two step Batch Anneal plus TemperMill route. In some instances, production of extra deep draw enamelingmaterial does not utilize in-line tempering on the CAL.

The specialized enameling alloy requires a restrictive carbon (C)tolerance (0.004-0.0100 wt %) which is needed for the production ofhighly formable non-sagging cold rolled steel and thereby making itsuitable for vitreous enameling applications. In addition, the newenameling alloy has restricted C, titanium (Ti), nitrogen (N), andsulfur (S) chemistries with inter-related chemical formulas orrelationships met in order to produce vitreous enamel steel of finelydispersed metal precipitates necessary for the production of highlyformable, outgassing-free enamel quality steel. The new enameling alloydescribed herein also has restricted manganese (Mn) and silicon (Si)contents in order to achieve desired mechanical properties andductility.

The highly formable enameling steel disclosed herein provides forproduction of enameling surface tolerances at cold rolling productionunits with no or minimal secondary surface roughening at the CAL. Assuch, a non-work hardened, highly formable steel with the necessarysurface qualities needed for efficiently bonding the enamel coatings tothe steel surface is provided.

The new alloy is a vacuum degassed steel chemistry comprising thefollowing composition in weight percent (wt %): C≦0.0100; Si≦0.0600;Mn≦0.17; P≦0.0100; 0.0200≦S≦0.0500; 0.0150≦Al≦0.0800; Cr≦0.1000;Cu≦0.1000; Nb≦0.0100; Mo≦0.0500; N≦0.0100; Ni≦0.0600; 0.0650≦Ti≦0.1500;B≦0.0004; Sn≦0.0150 and with the balance being Fe and incidentalresiduals/impurities commonly present in the steel making process. Inaddition, the alloy obeys the following enameling chemistry controlformulas (in wt %):

14.0≧[Ti/(C+N)]≧8.0  (1)

and

Ti_(free)=[Ti_((addition))−(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2)

in order to enhance the formation of finely dispersed metal precipitatesin the steel and achieve excellent enameling properties.

Carbon content has a strong influence on the strength and formingcharacteristics of the steel. For enameling steels in particular, theamount of carbon present in the steel should be sufficient to providestructural strength to deep drawn parts and to prevent sagging andwarpage during the manufacture of enamel products. If carbon iselevated, the strength of the steel is increased; however, theformability properties, particularly the plastic strain ratio or themechanical anisotropy, is impaired and vice versa. As such, it ispreferable to restrict the carbon level and thereby control the saggingstrength and forming properties of the steel. Therefore, a preferredembodiment has a carbon content less than or equal to 0.010.

Aluminum is added for de-oxygenation and to improve the surface qualityof the steel. In enamel steels, a small amount of excess free Al isdesirable for AlN formation and to eliminate premature ageing of thesteel. The amount of Al added to the alloy is carefully controlled sothat oxygen and free nitrogen are practically eliminated. Therefore, theAl content is kept between 0.0150 and 0.0800.

Silicon is added to increase the strength of steel. However, an elevatedSi can cause significant reduction of the ductility of the galvanizesteel. Therefore, Si content is limited to ≦0.0600.

Phosphorous has an effect for solid solution strengthening. Excessive Pin the presence of Al can cause a selective oxidation effect anddeterioration in surface quality, thereby rending the product unsuitablefor exposed surface application. Therefore, the preferred P content islimited to ≦0.0100%.

Manganese is a grain refiner and acts to strengthen the steel andprevent hot shortness in non-inventive conventional grades. Theinventive steel grade does not exhibit hot shortness at low Mn levels.The detrimental iron-sulfide precipitation, which exhibits elongatedsulfides after hot rolling, is prohibited by precipitation oftitanium-sulfides or complex precipitates of Ti, C, N, S. In this regardTi substitutes the role of Mn in non-inventive grades. If Mn is used inthe conventional range, the formability can be adversely affected due toincreased strength of the steel. As such, the Mn level is controlledbelow 0.17.

The inventive steel grade is manufactured by casting a slab or adirectly cast transfer bar with the disclosed chemical composition andhaving a thickness between 40 and 280 millimeters (mm), inclusive. Theslab or transfer bar is then soaked at a temperature greater than orequal to 1200° C. either in reheating a slab after it has cooled orhomogenization of a directly casted transfer bar. The slab or transferbar is then hot rolled using a roughing treatment at temperaturesbetween 900-1200° C. to produce a transfer bar with a thickness between20-70 mm, inclusive. The transfer bar is then subjected to a finishingtreatment in which additional hot rolling is performed. The finishingtreatment has an entry temperature between 900-1100° C. and an exittemperature between 760-960° C. Upon exiting the finishing treatment,the steel is in the form of hot rolled strip which can be formed orwound into a coil, i.e. coiled, at a temperature between 560-790° C. Inaddition, the hot rolled strip can have a thickness range between1.5-6.0 mm.

The hot rolled strip is pickled and cold rolled with a reduction insheet thickness ranging between 50 to 90% and with a specialized workroll (W/R) having a surface roughness ranging between 2 and 15 Ra, where‘Ra’ is the arithmetic average of a roughness profile for a surface.

The cold rolled sheet is continuously annealed in an annealing furnacestarting with slow heating from an entry coil temperature, e.g. ≦50° C.,to a strip soak temperature between 760-880° C. The sheet is thenisothermally soaked within this inter-critical soak temperature regionfor a soak time of up to 180 seconds, followed by cooling to ambienttemperature. The cooling may or may not be interrupted by holding in atemperature range between 250 to 600° C. or may even be fast cooled to atemperature below 250° C. and then reheated to a temperature rangebetween 250 to 600° C. before being finally cooled to ambienttemperature.

To produce the extra deep drawn enameling material, no temper rolling isperformed on the cold rolled annealed sheet. The required surfaceroughness is made at the cold rolling mill using the work rolls with a2-15 Ra surface roughness. For the deep drawn material, a temper rollingapplication in the amount of 0.3-2.0% is applied. It is permissible toincorporate additional temper rolling to the deep drawn material, ifflatness issues or surface quality issues appear on the cold rolledannealed sheet. However, it is appreciated that the additional temperrolling or tension levelling application can alter the surface roughnessof the material and ultimately affects the enameling properties.

In order to better teach the invention but not limit its scope in anymanner, specific examples are discussed. Ten slabs having chemicalcompositions shown in Table 1 were soaked at 1275° C. and subsequentlyhot rolled using a roughing treatment to produce transfer bars. The tenbars were subjected to a hot rolling finishing treatment and coiled toproduce hot band coils having a thickness between 5.0-6.0 mm.

TABLE 1 % C % Si % Mn % P % S % Cr % Mo % Ni % Al % Cu COIL 1 0.00620.006 0.1031 0.0102 0.0228 0.014 0.0009 0.0122 0.0391 0.0042 COIL 20.0067 0.005 0.1005 0.0084 0.0257 0.015 0.0011 0.0121 0.0345 0.0042 COIL3 0.0072 0.013 0.0952 0.0095 0.0237 0.018 0.0017 0.0055 0.0488 0.0103COIL 4 0.0075 0.013 0.096 0.0074 0.0263 0.015 0.0018 0.0056 0.04920.0087 COIL 5 0.0072 0.011 0.0959 0.0091 0.0246 0.016 0.0018 0.00550.0453 0.0097 COIL 6 0.0073 0.013 0.0963 0.0072 0.0262 0.014 0.00160.0055 0.0495 0.0086 COIL 7 0.0072 0.013 0.0975 0.0071 0.0260 0.0150.0019 0.0056 0.0499 0.0088 COIL 8 0.0070 0.012 0.0969 0.0092 0.02490.016 0.0017 0.0056 0.0457 0.0099 COIL 9 0.0072 0.013 0.0984 0.00700.0262 0.014 0.0018 0.0055 0.0507 0.0059 COIL 10 0.0064 0.011 0.09750.0099 0.0240 0.016 0.0018 0.0055 0.0473 0.0106 % Nb % Ti % V % B % N %Sn % Tifree Ti/(C + N) COIL 1 0.0019 0.154 0.0030 0.0000 0.0060 0.00070.0390 9.790 COIL 2 0.0018 0.1036 0.0027 0.0000 0.0039 0.0007 0.02509.870 COIL 3 0.0030 0.1049 0.0050 0.0001 0.0040 0.0005 0.0270 9.400 COIL4 0.0035 0.1087 0.0053 0.0002 0.0044 0.0006 0.0250 9.170 COIL 5 0.00320.1015 0.0050 0.0002 0.0040 0.0004 0.0250 9.140 COIL 6 0.0031 0.10980.0053 0.0002 0.0041 0.0007 0.0280 9.710 COIL 7 0.0039 0.1098 0.00550.0001 0.0042 0.0009 0.0280 9.670 COIL 8 0.0035 0.1021 0.0052 0.00010.0039 0.0007 0.0240 9.410 COIL 9 0.0036 0.1108 0.0058 0.0002 0.00410.0007 0.0290 9.590 COIL 10 0.0035 0.1023 0.0051 0.0002 0.0036 0.00080.0290 10.230

The hot band coils were then subjected to pickling and cold reduction tomake cold rolled full hard coils with a thickness of about 1.40 mm. Thecold reduction was a 75% reduction in thickness and the material had asurface roughness between 2.0-3.5 Ra.

After the cold reduction, the cold rolled full hard coils werecontinuously annealed in a CAL at 820° C. with a 75 second soak time,cooled to 600° C. in a first cooling step with an average cooling rateof 13 K/s, and further cooled to 120° C. with an average cooling rate of6 K/s before exiting the CAL furnace. After exiting the furnace, thecoils were further cooled to 40° C. and no temper rolling was applied onthe CAL. The material was then trimmed to width and samples were cut fortesting prior to recoiling and removal from the CAL.

The results of the mechanical and surface roughness testing for thesamples of inventive steel are shown in Table 2. All ten coils achievedor exceeded the desired mechanical properties for extra deep draw parts.Tables 3 through 7 show a comparison of the inventive steel's (Steel A)mechanical properties to a comparative enamel steel (Steel B) and acomparative interstitial free steel. It is appreciated that the“N-value” shown in Table 6 is the strain hardening exponent and the“R-value” is the Lankford coefficient or plastic strain ratio known tothose skilled in the art.

TABLE 2 LOWER ELONGATION SURFACE YIELD YIELD TENSILE UNIFORM AT SURFACEROUGHNESS, STRENGTH STRENGTH STRENGTH ELONGATION FRACTURE N- R-ROUGHNESS, BOTTOM (0.2% MPa) (MPa) (MPa) (%) (%) VALUE VALUE TOP (μm)(μm) COIL 1 124.53 124.53 300.49 26.10 48.99 0.250 2.088 2.63 2.68 COIL2 125.20 125.20 298.52 25.80 48.73 0.248 2.077 2.65 2.75 COIL 3 129.05129.05 308.15 25.22 48.37 0.250 2.070 2.02 2.21 COIL 4 129.51 129.51306.91 24.56 46.83 0.240 2.075 2.71 2.69 COIL 5 125.04 125.04 303.1225.06 48.07 0.247 2.082 2.64 2.75 COIL 6 131.63 131.63 305.86 23.7447.20 0.238 2.178 2.60 2.59 COIL 7 123.04 123.04 303.98 24.40 47.840.245 2.260 2.45 2.42 COIL 8 122.74 122.74 304.46 24.59 47.36 0.2452.168 2.59 2.52 COIL 9 123.43 123.43 304.01 25.75 48.42 0.250 2.113 2.472.46 COIL 10 123.84 123.84 301.59 25.61 49.01 0.250 2.182 2.45 2.40

TABLE 3 LOWER YIELD STENGTH (MPa) STEEL A STEEL B STEEL C Min 121.3140.9 127.85 Median 125.7 175.6 137.8 Max 132.6 179.8 161.5 Std 2.62111.396 10.958 N 48 10 190

TABLE 4 TENSILE STRENGTH (MPa) STEEL A STEEL B STEEL C Min 293.7 299.6285.15 Median 304.75 325.1 295.05 Max 312.6 327.2 303.75 Std 3.850 8.3004.008 N 48 10 190

TABLE 5 ELONGATION (%) STEEL A STEEL B STEEL C Min 46.1 43 47.7 Median48.15 43.65 50.475 Max 51.1 48.7 52.25 Std 1 2 1 N 48 10 190

TABLE 6 N-VALUE STEEL A STEEL B STEEL C Min 0.235 0.219 0.237 Median0.248 0.2235 0.2495 Max 0.252 0.248 0.256 Std 0.0036 0.0083 0.007 N 4810 190

TABLE 7 R-VALUE STEEL A STEEL B STEEL C Min 1.96 1.93 2.105 Median 2.1051.99 2.278 Max 2.54 2.22 2.64 Std 0.0962 0.0928 0.102 N 48 10 190

Selected mechanical properties from the inventive steel (Steel A), acomparative enamel steel (Steel B), and a comparative interstitial free(IF) steel (Steel C) are shown in FIGS. 1 and 2. Steel A and Steel Cexhibit greater formability than Steel B. Although Steel C showscomparable formability to Steel A, its chemistry values fall outside ofthe desired enameling chemistry control guidelines previously mentioned.Specific examples of the chemistry of Steel B and Steel C can be seen inTables 8 and 9, respectively. Similar to Steel A, Steel B's chemistryupholds the enameling chemistry control formulas, while Steel C'schemistry does not.

TABLE 8 % C % Si % Mn % P % S % Cr % Mo % Ni % Al % Cu COIL 1 0.00800.007 0.214 0.0115 0.0237 0.015 0.0001 0.010 0.0493 0.003 COIL 2 0.00800.007 0.214 0.0115 0.0237 0.015 0.0001 0.010 0.0493 0.003 COIL 3 0.00700.003 0.225 0.0175 0.0273 0.019 0.0011 0.011 0.0407 0.006 COIL 4 0.00700.003 0.225 0.0175 0.0273 0.019 0.0011 0.011 0.0407 0.006 COIL 5 0.00700.003 0.225 0.0175 0.0273 0.019 0.0011 0.011 0.0407 0.006 COIL 6 0.00700.003 0.225 0.0175 0.0273 0.019 0.0011 0.011 0.0407 0.006 COIL 7 0.00800.007 0.214 0.0115 0.0237 0.015 0.0001 0.010 0.0493 0.003 COIL 8 0.00700.003 0.225 0.0175 0.0273 0.019 0.0011 0.011 0.0407 0.006 COIL 9 0.00760.013 0.242 0.0122 0.0258 0.026 0.0001 0.008 0.0219 0.027 COIL 10 0.00630.006 0.232 0.0083 0.0259 0.011 0.0010 0.012 0.0296 0.004 % Nb % Ti % V% B % N % Sn % Ti_(free) Ti/(C + N) COIL 1 0.0004 0.1050 0.0019 0.00020.0032 0.0010 0.0266 9.375 COIL 2 0.0004 0.1050 0.0019 0.0002 0.00320.0010 0.0266 9.375 COIL 3 0.0010 0.1140 0.0031 0.0002 0.0036 0.00100.0328 10.755 COIL 4 0.0010 0.1140 0.0031 0.0002 0.0036 0.0010 0.032810.755 COIL 5 0.0010 0.1140 0.0031 0.0002 0.0036 0.0010 0.0328 10.755COIL 6 0.0010 0.1140 0.0031 0.0002 0.0036 0.0010 0.0328 10.755 COIL 70.0004 0.1050 0.0019 0.0002 0.0032 0.0010 0.0266 9.375 COIL 8 0.00100.1140 0.0031 0.0002 0.0036 0.0010 0.0328 10.755 COIL 9 0.0017 0.11400.0033 0.0001 0.0039 0.0010 0.0316 9.913 COIL 10 0.0002 0.0960 0.00100.0002 0.0030 0.0010 0.0217 10.323

TABLE 9 % C % Si % Mn % P % S % Cr % Mo % Ni % Al % Cu COIL 1 0.00160.001 0.129 0.0080 0.0059 0.006 0.0010 0.011 0.0348 0.007 COIL 2 0.00240.001 0.129 0.0086 0.0048 0.005 0.0010 0.008 0.0393 0.004 COIL 3 0.00180.001 0.132 0.0077 0.0046 0.007 0.0010 0.009 0.0404 0.005 COIL 4 0.00200.006 0.131 0.0096 0.0057 0.008 0.0010 0.007 0.0453 0.001 COIL 5 0.00150.004 0.138 0.0060 0.0067 0.005 0.0020 0.013 0.0335 0.013 COIL 6 0.00160.005 0.132 0.0082 0.0116 0.024 0.0020 0.012 0.0390 0.016 COIL 7 0.00130.001 0.120 0.0090 0.0124 0.010 0.0020 0.013 0.0314 0.014 COIL 8 0.00110.001 0.133 0.0078 0.0045 0.009 0.0020 0.009 0.0341 0.005 COIL 9 0.00160.001 0.127 0.0060 0.0074 0.009 0.0023 0.012 0.0366 0.013 COIL 10 0.00150.005 0.122 0.0102 0.0109 0.016 0.0020 0.012 0.0357 0.013 % Nb % Ti % V% B % N % Sn % Ti_(free) Ti/(C + N) COIL 1 0.0002 0.0780 0.0010 0.00010.0033 0.0010 0.0515 15.918 COIL 2 0.0002 0.0810 0.0010 0.0001 0.00290.0010 0.0543 15.283 COIL 3 0.0002 0.0520 0.0010 0.0001 0.0025 0.00100.0594 19.070 COIL 4 0.0002 0.0900 0.0010 0.0001 0.0018 0.0010 0.067323.684 COIL 5 0.0005 0.0820 0.0022 0.0001 0.0023 0.0010 0.0581 21.579COIL 6 0.0005 0.0770 0.0025 0.0001 0.0029 0.0010 0.0433 17.111 COIL 70.0005 0.0710 0.0031 0.0001 0.0029 0.0010 0.0373 16.905 COIL 8 0.00050.0840 0.0032 0.0001 0.0022 0.0010 0.0654 25.455 COIL 9 0.0005 0.07400.0025 0.0001 0.0026 0.0010 0.0477 17.619 COIL 10 0.0005 0.0680 0.00200.0001 0.0033 0.0010 0.0344 14.167

In addition to the mechanical property testing, the microstructure ofthe inventive steel (Steel A), as well as the comparative steels (bothSteel B and Steel C) were examined. In particular, FIGS. 3, 4, and 5 areoptical micrographs of Steel A, Steel B, and Steel C, respectively.Finely dispersed TiS precipitates are present in Steel A and Steel B,but are not seen in Steel C.

While the present invention has been presented and described herein withthe specific example embodiments, it is to be understood that numerousmodifications and variations can be made without deviations from thespirit and scope of the invention. As such, it is the claims, and allequivalents thereof, which define the scope of the invention.

We claim:
 1. A process for producing high strength steel, the processcomprising: providing a steel slab having a chemical composition inweight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr,0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 maxNi, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance beingFe and incidental impurities; the chemical composition in wt % of thesteel slab obeying the following formulas:14.0≧[Ti/(C+N)]≧8.0  (1)andTi_(free)=[Ti_((addition))−(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2) soaking thesteel slab at a temperature of at least 1200° C.; hot rolling the steelslab using a roughing treatment and producing a transfer bar; hotrolling the transfer bar using a finishing treatment and producing hotrolled strip; pickling and cold rolling the hot rolled sheet andproducing cold rolled sheet; annealing the cold rolled sheet in acontinuous annealing line (CAL), the annealed cold rolled sheet having asurface roughness between 2.0-3.5 Ra, a lower yield strength of at least110 MPa, a tensile strength of at least 280 MPa, an elongation tofailure of at least 40%, an N-value of at least 0.200, and an R-value ofat least 1.80.
 2. The process of claim 1, wherein soaked steel slab ishot rolled using the roughing treatment within a temperature range of900-1200° C.
 3. The process of claim 2, wherein the transfer bar has athickness between 20-70 mm, inclusive.
 4. The process of claim 3,wherein the transfer bar hot rolled in the finishing treatment has anentry temperature between 900-1100° C., inclusive, and the hot rolledstrip exiting the finishing treatment has an exit temperature between760-960° C., inclusive, and a thickness between 1.5-6.0 mm.
 5. Theprocess of claim 4, further including coiling the hot rolled strip at atemperature between 560-790° C., inclusive.
 6. The process of claim 5,wherein the cold rolled sheet has a reduction in thickness compared tothe hot rolled strip of between 50-90%, inclusive.
 7. The process ofclaim 6, wherein the cold rolled sheet is annealed in the CAL at a soaktemperature between 760-880° C.
 8. The process of claim 7, wherein thecold rolled sheet is held at the soak temperature for a soak time up to180 seconds and then cooled to an ambient temperature.
 9. The process ofclaim 8, further including interrupting the cooling of the cold rolledsheet to the ambient temperature and holding the cooling cold rolledsheet in a temperature range between 250-600° C. for a predeterminedtime after the cold rolled sheet has annealed at the soak temperaturefor the soak time.
 10. The process of claim 8, further including rapidlycooling the cold rolled sheet from the soak temperature to the ambienttemperature at an average cooling rate of up to 100 K/s and thenreheating the cooled cold rolled sheet to a temperature range between250-600° C. for a predetermined time.
 11. The process of claim 8,further including temper rolling the cold rolled sheet between 0.3-2.0%.12. A process for producing highly formable steel, the processcomprising: providing a steel slab having a chemical composition inweight percent (wt %) with a range of 0.0100 max C, 0.0600 max Si, 0.17max Mn, 0.0100 max P, 0.0200-0.0500 S, 0.0150-0.0800 Al; 0.10 max Cr,0.1000 max Cu, 0.0100 max Nb, 0.0500 max Mo, 0.0100 max N, 0.0600 maxNi, 0.0650-1500 Ti, 0.0004 max B, 0.0150 max Sn, with the balance beingFe and incidental impurities; the chemical composition in wt % of thesteel slab obeying the following formulas:14.0≧[Ti/(C+N)]≧8.0  (1)andTi_(free)=[Ti_((addition))−(4·C)−(3.4·N)−(1.5·S)]≧0.020  (2) soaking thesteel slab at a temperature of at least 1200° C.; hot rolling the steelslab using a roughing treatment within a temperature range between900-1200° C., inclusive, and producing a transfer bar having a thicknessbetween 20-70 mm, inclusive; hot rolling the transfer bar using afinishing treatment with an entry temperature between 900-1100° C.,inclusive, and an exit temperature between 760-960° C., inclusive, andproducing hot rolled strip with a thickness between 1.5-6.0 mm,inclusive; coiling the hot rolled strip at a temperature between560-790° C., inclusive; pickling and cold rolling the hot rolled sheetand producing cold rolled sheet; and annealing the cold rolled sheet ina continuous annealing line (CAL), the annealed cold rolled sheet havinga surface roughness between 2.0-3.5 Ra, a lower yield strength of atleast 110 MPa, a tensile strength of at least 280 MPa, an elongation tofailure of at least 40%, an N-value of at least 0.200, and an R-value ofat least 1.80.
 13. The process of claim 12, wherein the cold rolledsheet has a reduction in thickness compared to the hot rolled strip ofbetween 50-90%, inclusive.
 14. The process of claim 13, wherein the coldrolled sheet is annealed in the CAL at a soak temperature between760-880° C.
 15. The process of claim 14, wherein the cold rolled sheetis held at the soak temperature for a soak time up to 180 seconds andthen cooled to an ambient temperature.
 16. The process of claim 15,further including interrupting the cooling of the cold rolled sheet tothe ambient temperature and holding the cooling cold rolled sheet in atemperature range between 250-600° C. for a predetermined time after thecold rolled sheet has annealed at the soak temperature for the soaktemperature.
 17. The process of claim 15, further including rapidlycooling the cold rolled sheet from the soak temperature to the ambienttemperature and then reheating the cooled cold rolled sheet to atemperature range between 250-600° C. for a predetermined time.
 18. Theprocess of claim 15, further including temper rolling the cold rolledsheet between 0.3-2.0%.